The instant invention relates to strontium feldspar aluminum titanate ceramics. Specifically the invention relates to strontium feldspar aluminum titanate ceramics having a low thermal expansion and high thermal shock resistance, suitable porosity and a narrow pore size distribution for diesel exhaust filtration applications.
In the industry cordierite (2MgO.2Al2O3.5SiO2) has been the cost-effective material of choice for high temperature filtering applications, such as flow-through and wall-flow filters, due to its combination of good thermal shock resistance, filtration efficiency, and durability under most operating conditions. However, under certain circumstances cordierite filters are susceptible to damage and have even catastrophically failed. Occasional thermal runaway occurs during uncontrolled regenerations resulting in localized cordierite melting. Another factor contributing to failure occurs when metal impurities from the engine oil, catalytic additives or corroded metal from the exhaust port liners are introduced into the filter during operation. Typically, at temperatures exceeding 1300xc2x0 C., these metals form oxides which react with the cordierite structure. Evidence of the failed material are usually small holes on the filter where the metal initially deposits and reacts causing corrosion and melting of the material.
Recently, silicon carbide (SiC) has been used as an alternative to cordierite in the fabrication of diesel particulate filters. However, SiC is costly and must be segmented due to an inherent high coefficient of thermal expansion (CTE) and poor thermal shock resistance.
A need therefore exists for a ceramic suitable for high temperature applications without the shortfalls of cordierite, but with all its advantages. The present invention provides such a ceramic.
The instant invention is founded upon the discovery of a strontium feldspar aluminum titanate low expansion, high thermal shock resistant ceramic, hereafter referred to as SAT. The inventive ceramic comprises a strontium feldspar (SrO.Al2O3.2SiO2) (SAS) phase and an aluminum titanate (Al2TiO5) (AT) second phase, and is extrudable into honeycomb structures. Iron (Fe) or magnesium (Mg) substitution within the AT phase is found to improve the stability of the ceramic against decomposition, especially at high temperatures above 800xc2x0 C. In the present invention, isormorphic substitution of Fe and Mg for aluminum within the AT is limited to a range between about 0-60 mole percent (%), preferably 1-10 mole %.
Inventive structure has a formulation which consists essentially, by weight, of between about 50-90% Fe or Mg stabilized AT and between about 10-50% SAS. Advantages include low coefficient of thermal expansion, high thermal shock resistance, high volumetric heat capacity, high interconnected porosity and large median pore size.
Accordingly, the coefficient of thermal expansion (CTE) from room temperature (RT) to 1000xc2x0 C. is about xe2x88x9210xc3x9710xe2x88x927/xc2x0 C. to +15xc3x9710xe2x88x927/xc2x0 C., preferably from room temperature to 800xc2x0 C. of about xe2x88x920.5xc3x9710xe2x88x927/xc2x0 C. to +6xc3x9710xe2x88x927/xc2x0 C. The voltmeter heat capacity is greater than 3.2 J cmxe2x88x923Kxe2x88x921 as measured at 500xc2x0 C. The total porosity is about 15-50% by volume, preferably about 40-50% by volume, and, the median pore size is about 5-50 micrometers, preferably about 8-15 micrometers.
Preferably the inventive ceramic consists essentially of about 70-80% by weight Fe stabilized AT and about 20-30% by weight SAS, and has a coefficient of thermal expansion over a temperature range from room temperature to 800xc2x0 C. of about xe2x88x920.3xc3x9710xe2x88x927/xc2x0 C. to +2xc3x9710xe2x88x927/xc2x0 C., a porosity of about 40-45% by volume, and a median pore size of about 15-20 micrometers.
The inventive ceramic is suitable in high temperature applications such as wall-flow filters for diesel exhaust filtration, and automotive catalytic converters.
Wall-flow filters comprising the inventive structure exhibit high volumetric heat capacity, and low pressure drop across the length of the. The inventive ceramic is preferably extruded into a honeycomb structure having an inlet end and an outlet end and a multiplicity of cells extending from the inlet end to the outlet end, the cells having porous walls, wherein part of the total number of cells at the inlet end are plugged along a portion of their lengths, and the remaining part of cells that are open at the inlet end are plugged at the outlet end along a portion of their lengths, so that an engine exhaust stream passing through the cells of the honeycomb from the inlet end to the outlet end flows into the open cells, through the cell walls, and out of the structure through the open cells at the outlet end.